1. Field of Invention
This invention relates to halftones and hyperacuity printers.
2. Background of the Invention
High quality printing depends on many factors, including how much fidelity information can be passed to the printer from a data source or image generator. One purpose of printer electronics is to use image processing to convert high fidelity, sampled image data into a high-band width, high-resolution bit stream for delivery to a printer. Because many printers are binary, image processing operations such as halftoning and thresholding are sometimes used. The printer electronics can use these halftoning and thresholding algorithms to convert a rasterized image of grayscale pixels into a rasterized bitstream of high addressability binary on-off signals for a laser, for instance. These halftoning or thresholding algorithms can be complex mathematical algorithms, implemented with real time math processing, or computed in advance and stored in lookup tables for fast access.
A resampling interpolator is typically found in the electronic pathway between the image generator and the output of the printer. The resampling interpolator is customarily used to provide resolution conversion, but can also be used to electronically compensate for spatial misalignment of the printer. The latter is called electronic registration, and requires fractional positioning of the data sample between rows and columns of pixels in the image as the relative position of the scanning light beam is continuously monitored with respect to the image data. To obtain a resample, the current position of the light beam is measured in units of addressability in both an x (fastscan) direction and a y (slow scan or process) direction.
To implement electronic registration fully, even the halftoning process must be resampled at fractional intervals with respect the halftone dot grid. This is a problem because the halftone structure is typically restricted to integer grid positions to minimize moiré. Moiré is an error in intensity caused by uncompensated overlapping of the halftone grids with themselves or the output grid structure. In order to resample a halftone on a fractional grid, a special type of halftoner called an “irrational halftoner” can be used, where off-grid resamples are allowed. Unfortunately, irrational halftoning can cause an increase in undesirable moiré.
Irrational halftoners are also used to make better halftone screen systems, such as for a full color screen system. These screen systems, such as those that can produce classic “rosette” halftone structures, require the resampling of the halftone grid on fractional intervals because the halftone angle or frequency may be irrational with respect to the scanning grid. For instance, to produce a halftone angle of 30 degrees, a resampling grid slope involving the square root of three is unavoidable. Once again, this need for irrational halftoning for classic “rosette” halftoning structures causes an increase in undesirable moiré. Reference is made to U.S. Pat. No. 5,410,414, “Halftoning In A Hyperacuity Printer”, which is incorporated herein by reference in its entirety, for a full teaching of an irrational halftoner that is compatible with the moiré cancellation extension provided by the invention herein.